fix(interpreter): keep repl alive after child exit

[chaliy]

What

• stop nested script and bash -c exits from firing the host REPL on_exit hook
• keep EXIT-trap execution scoped to the child shell context
• report a bash-compatible BASH_VERSION and BASH_VERSINFO surface so bash feature gates keep working under bashkit
• preserve BASH_VERSINFO for path-executed shebang scripts, which run in isolated child shell state
• add regression coverage for direct script execution, PATH-searched scripts, nested shells, sourced files, child EXIT traps, bash-version compatibility, and shebang-script subprocesses

Why

hs --help can call exit 1 inside the harness script when no help or agent command exists. Bashkit was treating that child exit as a whole-session exit in interactive mode, so the REPL closed instead of returning a failing command status.

While validating the fix, CI exposed two bash compatibility bugs in the harness example path:

• bashkit reported its crate semver through BASH_VERSION and BASH_VERSINFO
• path-executed shebang scripts dropped BASH_VERSINFO entirely when subprocess isolation cleared child shell arrays

How

• split interpreter script execution between EXIT-trap handling and host on_exit hook notification
• disable host exit-hook notification for nested shell/script execution paths
• preserve host exit-hook behavior for top-level exit and source
• replace the crate-version-based bash version surface with a fixed bash-compatible tuple while keeping the machine slot virtual
• reseed BASH_VERSINFO when path-based script execution resets child shell state to exported env only

Tests

• added focused hook regressions in crates/bashkit/src/lib.rs
• added bash-version compatibility regressions in crates/bashkit/src/lib.rs
• added path-script shebang regressions in crates/bashkit/src/lib.rs
• cargo test -q test_bash_versinfo_reports_bash_compatible_major --lib
• cargo test -q test_bash_version_surface_matches_bash_compatible_tuple --lib
• cargo test -q test_path_script_retains_bash_versinfo_array --lib
• cargo test -q test_path_script_bash_versinfo_satisfies_bash4_guard --lib
• cargo test -q test_on_exit_hook --lib
• cargo test -q test_path_script_exit_runs_child_exit_trap --lib
• just pre-pr

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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